Javascript: hide fix elements when add a big cover element on the page

hide_fix.js

var res = [];

/**
 * Try to hide elements with style fixed
 */
function hide_fix() {
	var possibilities = ['[style*="position:fixed"],[style*="position: fixed"]'],
		searchFor = /\bposition:\s*fixed;/,
		cssProp = 'position',
		cssValue = 'fixed',
		styles = document.styleSheets,
		i, j, l, rules, rule, elem;

	for (i = 0; i < styles.length; i++) {
		rules = styles[i].cssRules;
		if(!rules) return;
		l = rules.length;
		for (j = 0; j < l; j++) {
			rule = rules[j];
			if (searchFor.test(rule.cssText)) {
				possibilities.push(rule.selectorText);
			}
		}
	}
	possibilities = possibilities.join(',');
	possibilities = document.querySelectorAll(possibilities);
	l = possibilities.length;
	for (i = 0; i < l; i++) {
		elem = possibilities[i];
		// Test whether the element is really position:fixed
		if (window.getComputedStyle(elem, null).getPropertyValue(cssProp) === cssValue) {
			res.push(elem);
		}
	}

	for (i = 0, l = res.length; i < l; i++) {
		res[i].setAttribute('data-style', res[i].style.visibility);
		res[i].style.visibility = 'hidden';
	}
}

/**
 * Recovery the elements style which are be marked with style fixed.
 */
function show_fix() {
	var i, l;
	for (i = 0, l = res.length; i < l; i++) {
		res[i].style.visibility = res[i].getAttribute('data-style');
	}
}